US President Donald Trump has taken Senate Republicans to task for failing to reach agreement on overhauling Obamacare, as a new report showed 32 million Americans would lose health insurance if the law was repealed without a replacement.

Trump gathered 49 Republican senators on Wednesday for a White House lunch after a bill to repeal and replace the 2010 Affordable Care Act collapsed on Monday following dissent from a handful of the party's conservatives and moderates.

After Trump's exhortation, party members met Vice-President Mike Pence on Capitol Hill to try to come together on a major Republican campaign promise for the past seven years - undoing former president Barack Obama's signature legislation.

After taking a hands-off approach last week and suggesting on Tuesday he was fine with letting Obamacare fail, Trump demanded senators stay in Washington during the August recess until they found common ground.

"We can repeal but we should repeal and replace, and we shouldn't leave town until this is complete," Trump said at the meeting.

Even with Trump's new push, Senate leaders faced a difficult task getting moderates and conservatives to agree on an overhaul that could pass.

Senate Majority Leader Mitch McConnell had planned to hold a straight repeal vote next week but several Republican senators have already said they oppose that approach.

Senator John McCain's absence due to health issues, including a diagnosis of a brain tumour, has added to McConnell's vote-counting troubles.

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Thirty-two million Americans would lose their health insurance by 2026 if Obamacare is scrapped without an alternative in place, the non-partisan Congressional Budget Office reported on Wednesday, while 17 million would become uninsured next year alone.

At the same time, premiums on individual insurance plans would double by 2026.

"President Trump and Republicans have repeatedly promised to lower premiums and increase coverage, yet each proposal they offer would do the opposite," Senate Democratic leader Charles Schumer said.